Solid state photomultiplier arrays, for example, silicon photomultiplier (SiPM), are being developed because of the many applications in which they are proving to be able to offer unique advantages compared to other types of photo sensors usable for detection of even a relatively limited number of photons. Single photon avalanche diodes (SPAD), also known as Geiger-mode avalanche photodiode, are devices, typically based on a P+PN type junction (or a N+NP+ junction), useful for detection of single photons. The junction has a breakdown voltage VB and is biased, in use, with a reverse bias voltage VA higher in magnitude than the breakdown voltage VB of the junction, typically higher by 10-30%. In this way, the generation of a single electron-hole pair, following absorption of a photon impinging on the SPAD, is sufficient for triggering an ionization process that causes an avalanche of the carriers, with gains of around 106 and consequent generation in short times (hundreds of picoseconds) of an avalanche current that is collected through anode and cathode contacts.
Sensitivity and gain are directly proportional to the value of reverse bias voltage VA applied to the SPAD. In fact, the more the reverse bias voltage VA exceeds, in magnitude, the breakdown voltage VB, the higher the likelihood of an avalanche generation of charge carriers occurring.
However, a high reverse bias voltages VA may favor spurious triggers of an avalanche-ionization process, even in the absence of incident photons (dark conditions), by a single charge carrier that may be generated, for example, by thermal energy exchange, thus producing a so-called dark current. Moreover, a biasing voltage VA appreciably higher than the breakdown voltage VB may, once triggered, render the avalanche-ionization process self-sustaining, thus no longer capable of detecting arrival of successive photons.
Quenching of the avalanche current by lowering the effective voltage Ve across the junction to stop the avalanche-ionization process for a hold-off-time as brief as possible is a way of recovering the ability of detecting arrival of a next photon. A passive type quenching circuit comprises a quenching resistor of few hundred kilo-ohms in series with the junction.
Throughout this disclosure, reference is made to devices realized either on a p-type substrate or on a n-type substrate, for avoiding duplications and simplifying description; however, all the considerations made hold similarly for devices realized on a substrate of the opposite type of conductivity, i.e. inverting all the polarities and symbols in the attached drawings and exchanging expressions such as anode in lieu of cathode and vice versa, in the accompanying description, as will be recognized by the skilled person.
Commonly, a SiPM is a monolithically integrated circuit device of relatively large area and high gain, that basically comprises a dense array of microcells, each defining a SPAD with an associated quenching resistor, capable of supplying, on average, an electrical output signal (current) proportional to the number of photons that impinge on the SiPM. In fact, since the quenching resistors are uncoupled from one another, each photodiode SPAD of the SiPM behaves as an independent binary counter, while the output signal of the SiPM is proportional to the number of activated microcells, i.e. the number of SPADs through which an avalanche ionization process is triggered by a photon, this number being in turn proportional to the number of incident photons.